1. Field of the Invention
This invention relates to time of arrival detector circuits, and more particularly to such circuits in which an arriving signal is delayed and the delayed signal is compared with the original signal to determine its time of arrival.
2. Description of the Related Art
Time of arrival detectors for electrical pulses are used in applications such as radar and commercial pulsed ranging systems. The detection of a pulse's arrival time is complicated by the fact that pulses often vary over a wide range of amplitudes, rise and fall times, and pulse widths.
A pulse's time of arrival is measured with respect to some fixed time on the pulse, which ideally should be independent of rise and fall times for the pulse amplitude. The problem is illustrated in FIGS. 1a and 1b. Assume, for example, that a transmitter produces a single pulse that is received by two receivers, each equidistant from the source, but one having a higher gain antenna than the other (the analogous case of a single receiver and two transmitters is also possible). The receiver with the higher gain antenna will thus observe a greater amplitude pulse than the other receiver. In FIG. 1a the curve 2 represents the leading edge of the higher amplitude pulse, while in FIG. 1b the curve 4 represents the leading edge of the lower amplitude pulse.
One approach to determining the pulse's time of arrival would be to use a simple level detector to determine when the pulse crosses a fixed threshold, with the crossing point taken as the time of arrival. In FIG. 1a the larger amplitude pulse is illustrated as rising to a normalized unity peak amplitude. Assuming that the threshold level for the time of arrival indication is 0.5 and that the first energy of the incoming pulse is detected at time T1, a pulse arrival indication will be produced at time T2 when the rising pulse waveform crosses the 0.5 threshold. If the same 0.5 threshold is used for the same pulse in the lower antenna gain amplifier of FIG. 1b, which is assumed to have a peak value of only 0.6, the lower gain pulse 4 will cross the threshold at a time T2' that is later than T2.
The difference between T2 and T2' represents an error in the time of arrival measurement, since the two receivers are assumed to be equidistant from the transmitter and accordingly the pulse arrives at both receivers at the same time. To avoid amplitude errors with a simple level detector, the risetime of the pulse must be extremely short. Even worse, if the arriving signal is too weak to cross the threshold, it will never be counted as having "arrived".
To eliminate these problems, a method has been devised in which a delayed, amplified version of the pulse is compared to an undelayed, unamplified version to produce a time of arrival indication. With this approach, the time of arrival is designated as the time at which the amplified version of the pulse reaches a value that is a fixed amount below its final value, such as 3dB, less the delay that was added to the amplified signal. The amplification factor is accordingly set so that the peak value of the undelayed, unamplified pulse is 3dB less than the peak value of the delayed amplified pulse. The unamplified pulse can thus be used as a threshold, with the time of arrival based upon the time when the delayed and amplified pulse rises above the undelayed, unamplified pulse. This technique is described, for example, in Tsui, Microwave Receivers with Electronic Warfare Applications, John Wiley & Sons, 1986, pages 89-94, and is illustrated in FIGS. 2a and 2b.
In FIG. 2a the original unamplified pulse 6 is again assumed to begin rising at time T1. The amplified version 8 of the pulse begins to rise after a fixed delay period, and continues rising to a level 3dB above the peak of the undelayed pulse 6. The time T2 at which the delayed pulse 8 rises above the undelayed pulse 6 is taken as the time of arrival for the original pulse (minus the delay).
FIG. 2b demonstrates that, with the described approach, two pulses that arrive at the same time and have equal rise times will result in simultaneous time of arrival indications even if their amplitudes differ. In FIG. 2b a similar pulse 6', which is an attenuated version of pulse 6, also begins to rise at time T1 and has the same rise time as pulse 6. A 3dB amplified and delayed pulse 8' that corresponds to delayed pulse 8 in FIG. 2a can be seen to cross and rise above the undelayed pulse 6' at the same time T2 as in FIG. 2a.
A block diagram of circuitry that is commonly used to implement the time of arrival scheme of FIGS. 2a and 2b is shown in FIG. 3. The input signal Vin is applied to an input terminal 10. The input line then branches, with one branch 12 connected directly to the negative input of an analog comparator CMP1. The other branch 14 is connected to a line driver amplifier A1 which drives the input signal through a delay line 16, typically a 40 nsec delay. An amplifier A2 at the output of the delay line overcomes the line loss and amplifies the delayed signal, generally by 3dB. The amplified delayed signal is applied to the positive input of comparator CMP1 for comparison with the unamplified, undelayed signal. The comparator produces a time of arrival indication at output terminal 18 when the signal at its positive input rises above the signal at its negative input.
The delay line 16 must be long enough to accommodate input signals with slow rise times, but not so long that its delay exceeds the width of narrow pulses. To measure pulses as narrow as 50 nsec or as wide as 200 microsec, a reasonable compromise for the delay period is on the order of 40 nsec. Additionally, the delay line must be wideband, since rise and fall times can be as small as 10 nsec Wideband coaxial cable is typically used, with approximately 28 linear feet of cable required to achieve the desired 40 nsec delay. This delay line has an impedance on the order of 50 ohms, and requires a relatively high power driver to transmit the signal (typically a few volts in amplitude). This delay line is large and bulky, and its delay tends to vary with frequency and temperature. The long length of the delay circuit makes it difficult, if not impossible, to miniaturize a circuit using this technique. Although ultraminiature coaxial cable is available with diameters as small as 0.2 mm, the resulting line loss can be as high as 38 dB at 500 MHz, which is generally intolerable. The line driver itself needs to be wideband and capable of driving a large signal down a relatively low impedance transmission line at high speed, which often results high driver power dissipation. In some cases the delay line is reduced in size by increasing its capacitance per unit length. However, since the inductance per unit length usually remains the same, this technique results in very low values of line impedance, further increasing the driver power requirement.
Currently available delay lines and their associated circuitry cannot be implemented as monolithic integrated circuits due to the size limitations described above. They are therefore usually fabricated as hybrid integrated circuits, or as part of a printed circuit board assembly. In either case the system's manufacturability and reliability suffer because of the many individual components that require interconnection.